At present both ammonium paratungstate and ammonium metatungstate are being used as tungsten sources in the catalyst industry. Since ammonium paratungstate has a limited solubility in water (about 2 to 3% at room temperature), the user converts ammonium paratungstate to a more soluble peroxide in order to prepare the catalyst. While ammonium metatungstate is highly water soluble and thus need not be converted, it is in general more expensive than ammonium paratungstate due to the complexity and/or low yields of processes for producing it.
U.S. Pat. No. 3,175,881 describes a process for producing crystalline ammonium metatungstate from ammonium paratungstate. According to this process, ammonium paratungstate is heated at about 500.degree. F. to about 550.degree. F. for about 1 hour and digested deionised water to form a solution containing about 9 to about 10% dissolved solids and some insolubles, the solution digested and evaporated to about one third of its original volume while the pH is maintained at about 3.5 in order to effect the conversion of ammonium paratungstate to ammonium metatung state, and the insolubles removed. Ammonium metatungstate is then crystallized from the resulting solution.
U.S. Pat. No. 3,472,613 describes a process for preparation of ammonium metatungstate. According to this process, a water slurry of ammonium paratungstate is adjusted to a pH of about 1.75 to about 3.5 with a mineral acid such as nitric acid, hydrochloric acid sulfuric acid or an organic acid such as acetic acid, lactic acid or formic acid. The slurry is maintained at from about 50.degree. C. to about 120.degree. C. until the ammonium paratungstate is substantially completely converted to ammonium metatungstate. Ammonium metatungstate is then crystallized from the resulting solution.
The ammonium metatungstate produced in the above processes can be contaminated particularly with the anion of the acid which was used to adjust the pH.
A process for producing ammonium metatungstate which is relatively pure especially with respect to the anion of the acid used to adjust the pH would be an advancement in the art.